In driving a motor, one concern is the generation of noise at certain frequencies. For example, FIG. 1 shows an example of a conventional noise suppression circuit. FIG. 1 shows a motor 1000 having brushes 1012, 1014. Inductors 1020, 1022 are connected to each of brushes 1012, 1014. An “X capacitor” 1034 connects the motor terminals 1050, 1052, and “Y capacitors” 1030, 1032 connect motor terminals 1050, 1052 to ground.
As seen in FIG. 2, radiated noise is produced due to RF current 1100 on the wiring harness 1110 resulting from capacitance between the harness 1110 and the motor housing 1010. At low frequencies, Y capacitors 1030, 1032 short circuit this voltage to suppress the noise (see current 1102 in FIG. 2). However, it has been found that at high frequencies (e.g., above 200 MHz), the connection to the motor housing 1010 results in a higher level of RF noise than would exist if there was no connection to the motor housing 1010. It is presumed that this higher level of noise is due to resonance of current 1100 and 1102.
Removing Y capacitors 1030, 1032 to eliminate the connection to the motor housing 1010 would remove this resonant radiated noise at high frequencies (see FIG. 3). However, the connection to the motor housing 1010 must be maintained to suppress low frequency noise.
Additionally, FIG. 4 shows an example of a conventional Printed Circuit Board (PCB) trace layout for a noise suppression circuit. For example, traces 1200, 1202, and 1204 in FIG. 4 all have large widths to keep impedance low. However, when large width traces are adjacent to each other in a PCB layout, there is a possibility of high frequency shorting paths occurring due to capacitance between the traces. This high frequency shorting path can have implications in noise suppression when trying to implement a solution to the problem of high frequency radiated noise.
It may be possible to use feed-through capacitors to help suppress noise in some applications. However, it is important to note that the size of feed through capacitors make them unsuitable for use in small motors. Additionally, the cost of feed-through capacitors is prohibitive for widespread use.
Thus, there is a need for a reasonable cost structure that maintains connection to the motor housing at low frequencies and also suppressing radiated noise at high frequencies, while accounting for the possible issues caused by high frequency shorting paths in a PCB layout with large trace widths.